How to find it : enable Harmattan SDK, then "apt-cache show libmpfr1ldbl", the result has the line :
Filename: pool/harmattan/free/m/mpfr/libmpfr1ldbl_2.4.1-0maemo4+0m6_armel.deb
So, go to the Harmattan SDK repository site.

ok i found it (i modified my first response) but i can't execute make : not found

It *might* be worth it to use a chroot to ensure dependencies don't get bungled up.

i try a compilation but i have this response
and the solution was : ln -s /usr/bin/gcc-4.4 /usr/bin/gcc

Code:

~/compile/lame-3.99 $ ./configure
checking build system type... armv7l-unknown-linux-gnu
checking host system type... armv7l-unknown-linux-gnu
checking for a BSD-compatible install... /usr/bin/install -c
checking whether build environment is sane... yes
checking for a thread-safe mkdir -p... ./install-sh -c -d
checking for gawk... no
checking for mawk... no
checking for nawk... no
checking for awk... awk
checking whether make sets $(MAKE)... yes
checking whether to enable maintainer-specific portions of Makefiles... no
checking for style of include used by make... GNU
checking for gcc... no
checking for cc... no
checking for cl.exe... no
configure: error: in `/home/user/compile/lame-3.99':
configure: error: no acceptable C compiler found in $PATH
See `config.log' for more details

The /tmp on the device is too small though for anything of size. The normal way to address this to set the environment variable TMPDIR to point to a larger directory. But this doesn't work. Has anybody else seen this problem?